This invention relates to novel polymer blends which exhibit high impact strength, good retention of impact strength after heat aging, low flammability and good chemical resistance. More particularly, the invention relates to novel polycarbonate based polymer blends.
Polycarbonates are well-known polymers which have good property profiles, particularly with respect to impact resistance, electrical properties, optical clarity, dimensional rigidity and the like. Polycarbonates are high performance engineering thermoplastics, which are conveniently formed into a wide variety of useful articles by such techniques as extrusion, thermoforming, injection molding and blow molding.
One disadvantage of polycarbonates for certain applications is their relatively low critical thickness values, i.e., the thickness at which a discontinuity in Izod impact values occurs. For example, whereas typical notched Izod impact values for 3.2 mm thick test specimens are generally in the range of about 87 kgf-cm/cm, typical notched Izod impact values for 6.4 mm thick specimens are generally in the range of about 11 kgf-cm/cm. The relatively high notched Izod impact strengths of the 3.2 mm thick specimens are attributable to their being thinner than the critical thickness of the polymer, and therefore, upon impact, a hinged or ductile break occurs. The low notched Izod impact strength of the 6.4 mm thick specimens results from these specimens being thicker than the critical thickness of the polymer, and therefore, upon impact, a clean or brittle type break occurs. The low critical thickness values tend to limit the practical wall thickness of molded polycarbonate articles.
A further disadvantage often observed in articles fabricated from polycarbonates is their susceptability to environmental stress cracking and crazing. Environmental stress cracking and crazing refer to types of failure which are hastened by the presence of organic solvents, such as gasoline, acetone, heptane, carbon tetrachloride, and the like when such solvents are in contact with stressed parts fabricated from polycarbonates. The most significant effects are losses in impact strength and ductility.
Numerous polymers and additives have been blended with polycarbonates to improve the impact strength, environmental stress crack resistance and other properties of the polymers. To name but a few, the following polymers have been blended with polycarbonates to improve impact strengths and environmental stress crack resistance: polyacrylates (U.S. Pat. No. 4,616,042), polyolefins (U.S. Pat. No. 4,616,042), rubbery dienic polymers (U.S. Pat. No. 4,616,042), styrenic polymers (U.S. Pat. No. 4,616,042), graft modified polyolefins (U.S. Pat. No. 4,632,962), brominated polystyrene (U.S. Pat. No. 4,629,759), amorphous polyesters (U.S. Pat. No. 4,628,074), ethylene-propylene-dieneterpolymers (U.S. Pat. No. 4,626,572), hydrogenated AB diblock copolymers (U.S. Pat. No. 4,584,338), acrylate-styrene-acrylonitrile terpolymer and poly(methyl methacrylate) (U.S. Pat. No. 4,579,909). In addition, various silicone rubbers, silicone polysiloxane polymers and polyorganosiloxanes have been blended with polycarbonates to improve impact strength, chemical resistance and the like. See, e.g., U.S. Pat. Nos. 4,587,298, 4,536,590 and 4,335,032. Further, polydiorganosiloxane-polycarbonate block copolymers have been described as having improved impact strengths as compared to the unmodified polycarbonates. (See U.S. Pat. No. 4,616,042.)
Copending U.S. patent application Ser. No. 925,915, filed Nov. 3, 1986, discloses flame resistant polymer blends containing a polyetherimide, a siloxane-polyetherimide copolymer and a minor proportion of a polycarbonate. The siloxane-polyetherimide copolymer is said to improve the impact strength and processability of the polyetherimide, and the addition of the polycarbonate component is said to impart good chemical resistance to the resulting blends.
In the search for engineering thermoplastics capable of high performance applications, there is a continuing interest in developing polycarbonate based resins which have the advantageous physical and chemical properties of polycarbonates, yet have improved impact performance, particularly thick section impact strength, and resistance to environmental stress cracking and crazing.